Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Data Matrix, typically used for labeling small electronic products; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Bar codes may be one dimensional (1D), i.e., a single row of graphical indicia such as a UPC bar code or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single bar code, such as Data Matrix which comprising multiple rows and columns of black and white square modules arranged in a square or rectangular pattern.
Systems that read bar codes (bar code readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Bar code readers that read and decode bar codes employing imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging sensor arrays having a plurality of photosensitive elements (photosensors) defining image pixels. An illumination apparatus or system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a system of one or more lens of the imaging system onto the sensor array. Thus, the target bar code within a field of view (FV) of the imaging lens system is focused on the sensor array.
Periodically, the pixels of the sensor array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
As mentioned above, imaging-based bar code readers typically employ an imaging lens assembly for focusing scattered/reflected light from the field of view (FV) onto the sensor array. If a target object is within the field of view FV, an image of the target object will be focused onto the sensor array.
There are typically two types of imaging lens assemblies: 1) fixed focus lens systems; and 2) variable focus lens systems. In a fixed focus system, the field of view (FV) and a working range (WR) of the imaging system is fixed. The working range (WR) of an imaging system is a distance range in front of or forward of the imaging lens assembly within which a target object of interest, such as a target bar code, may be successfully imaged and decoded by the imaging system decoding circuitry.
The working range (WR) and field of view (FV) require a user to move the bar code reader relative to the target bar code such that the target bar code is within the field of view (FV) and within the working range (WR) of the imaging system for successful decoding of the imaged target bar code. At the near and far limits of the working range (WR), there is a problem with blurriness, that is, poor resolution of the imaged target bar code. A variable focus lens system addresses the blurriness problem at the limits of the working range (WR) by providing for readjustment/refocusing of the lens system. This readjustment/refocusing of the lens system facilitates obtaining an in-focus image of a target bar code focused onto the sensor array, thus, mitigating blurriness at the limits of the working range (WR).
Known mechanical variable focus lens assemblies depend on mechanical movement of one lens (or more lenses depending on the assembly) of lens assembly with respect to a stationary sensor array. In order to utilize an variable focus lens assembly, it is essential to know a reference position for the moving lens corresponding to a known or reference focal plane position, that is, for a given focal plane position, say reference focal plane position F, it is necessary to know a reference point or position of the moving lens along its path of travel, say lens reference point X. What this means is that when the lens is at reference point X and a target object is positioned at the given or reference focal plane position (say reference focal plane F), an image of the target object will be in-focus on the sensor array, that is, the best resolution image of the target object will be projected onto the sensor array.
It is important to know in advance the location of the reference point and reference focal plane for rapid acquisition of a decodable image of a target object. In some autofocus imaging systems, in order to obtain a decodable image of an encoded target object, the system moves the moving lens from one end stop and continues to iteratively analyze image frames until a decodable image frame is found. If it turns out that the first decodable image frame is somewhere around the middle of the moving lens path of travel, it is easy to see that this type of autofocusing system would, on the average, be slow in acquiring a decodable image of the target object, thus, causing the system to be perceived as sluggish.
By finding a reference point along the path of travel and positioning the moving lens at the reference point for each reading session, instead of having to move the moving lens over an extended portion of the path of travel, the focusing system can quickly move from the reference position to a position where an image of the target may be successfully decoded assuming that the target object is located within the field of view somewhere generally close to the reference focal plane.
Another reason that it is important to be able to establish a precise reference point or position for the moving variable focus lens is because, over time, systemic changes in the moving lens drive system (such as, for example, weakening of bias springs or the wearing down of mechanical surfaces in the case of a mechanical variable focus system) may cause shifting of a previously established reference position for the moving lens. These types of systemic changes in a previously established lens reference position is called a bias error and is of a much more serious nature than differential errors, that is, errors that result in inaccuracies of from moving the lens one position to another along its path of travel. Additionally, variable focus lens assemblies typically do not provide a mechanical stop that could be used as a moving lens reference point because such a mechanical stop along the moving lens path of travel may not be possible or, if possible, may be difficult and costly to implement. Further, while such mechanical stops may provide accurate positions for starting and ending positions along the moving lens path of travel, such mechanical end stops do not provide any feedback to locate the lens in an intermediate position. Locating the lens in an intermediate position facilitates rapid acquisition of an in-focus image of a target object, as discussed above.
In addition to mechanical variable focus lens assemblies discussed above, another type of variable focus lens assembly utilizes a liquid lens. A liquid lens is lens formed by two immiscible liquids of differing conductivity sandwiched between two windows. As an electrical field applied to the liquids is varied, the shape of the interface between the liquids changes and thereby changes the optical characteristics of the liquid lens. Typically, a voltage is applied to the liquid lens, as the applied voltage varies, the optical characteristics change.
Accordingly, in a liquid lens, the “reference point” would be a reference voltage value applied to the liquid lens. The referent point voltage value is the voltage value that corresponds to a reference focal plane. Instead of moving a moving lens along a path of travel to find a reference point along a path of travel of the lens, with respect to a liquid lens, the reference point voltage value is a specific applied voltage value in a range of voltages within an operating range of the liquid lens.
The need to establish a reference point voltage for a reference focal plane is due to the fact that the relationship between the optical power of the liquid lens and the applied voltage depends on temperature and may vary over time. Therefore, it is important to periodically calibrate the image system with respect to a reference focal plane.
What is needed is a method and apparatus for quickly and accurately locating a reference point for a variable focus lens of a variable focus imaging, lens assembly corresponding to a reference focal plane.